Art of uniting metals



Patented Sept. 28, 1937 ART OF UNITING METALS William J. Weder,Philadelphia, 2a., asslgnor to General Electric Company, a corporationof New York N Drawing.

6 Claims.

This invention relates broadly to the art ofuniting metals, and moreparticularly to improvements and modifications'in the art of unitingmetallic elements and alloys thereof which can be electroplated withmercury. It is more particularly concerned with, and. has as a mainobject to provide certain improvements in the process set forth in myco-pending application Serial No. 90,882, filed July 16, 1936, andassigned to the same assignee as the present application, which "is acontinuation-in-part of aforesaid co-pending application;

Briefly described, the present invention involves electroplating mercuryupon. at least one J of two facing surfaces of metals to be united andthen applying pressure, or preferably simultaneously applying heat andpressure, to bond themetals. In the preferred embodiment of theinvention excess mercury is vaporized from the surfaces during a heatand pressure treatment. My invention provides articles of manufacture,comprising metals, such as copper, silver and the like, rigidly andstrongly joined together by a process that is rapid, economical andeffective. The invention is especially applicable to uniting silver tocopper and its alloys/ hereinafter for brevity collectively designatedas copper, and for bonding copper to copper.

Before the action of current on metallic solutions was known (by meansof which certain metals can be plated from solutions on a given surface)amalgams were used for gilding and silvering. The article to be coatedwas covered by the amalgam and the excess mercury was volatilized by theapplication of heat, the gold or silver remaining on the surface as, astrongly adhering coating. Later, it was proposed. to apply heat andpressure simultaneously to the mercury-coated surfaces to form a unionof the metals. Unions of lead to lead, tin to tin, lead to tin, oreither lead or tin to any other metal which may be provided with atinned surface, are old and well known.

In my aforesaid co-pending application Serial No 90,882, I havedisclosed and claimed a method of uniting metal articles the surfaces ofwhich will form an amalgam with mercury which comprises applying a filmof mercury to the clean surfaces to be united, bringing the amalgamatedsurfaces into intimate contact with each other to join the articles,treating the joined articles for the removal of any entrapped gas,cushioning the joined articles tocompensate for any sur-- faceirregularities therein, and subjecting the 55 cushioned articles to asimultaneous pressure and with the copper.

Application April 21-, 1931, Serial No. 139,201

heat treatment sufiicient to remove excess mercury and firmly andpermanently unite the articles without detrimentally affecting thephysical properties thereof. In'that co-pending application products ofsuch process also were claimed.

The first steps of the process disclosed in -my co-pending applicationSerial No. 90,882 are hereinafter described more fully (with,particularreference, for purpose of illustration, to the bonding of silver tocopper), so that the differences between the present invention and theaforesaid co-pending application may be more easily understood. Theprocess disclosed in said co-pending application comprises the followingsteps:

1. The surface of the copper article, for example, a piece of flatcopper, was treated to render it substantially clean, said treatmentcomprising,for example, acid dipping or scrubbing with, for instance,pumice. The part of the copper to which the silver was to be applied,was coated with mercury as described in the following paragraph.

2. Over an area of the copper surface a little larger than the piece ofsilver to be applied, a small amount of an aqueous solution of mercuricnitrate and sodium cyanide was swabbed with a pad made of chamois skin,or metallic mercury and'sodium cyanide could be similarly applied, ineither case subsequently rubbing the surface until the mercury wasthoroughly amalgamated In using metallic mercury, it

was filtered through a chamois skin as it was applied to the coppersurface. The surplus mercury and cyanide solution were removed and thearticle set'aside to dry for about one-half hour. The resultant surfacewas polished with a. dry pad of'chamois skin. Next, fresh mercury wasapplied as has been described and the surplus was subsequently removed.g

3. A strip ofi'silver of the desired thickness, for example, 3mils-thick, and of theshape and size required, was brushed or wipedclean, and the I cleaned silver strip was then immersed in a bath ofmetallic mercury until it was coated. The amalgamated silver strip wasremoved from the mercury bath and brushed to remove excess mercury andany particles of dirt or other foreign impurities adhering thereto.

4. The mercury-coated silver strip was now placed upon the previouslymarked-ofi spot onthe mercury-coated copper surface, and pressed andfixed thereon by hand. It adhered readily to the amalgamated coppersurface.

5. The assembled article, if flat, was now pressed slowly andwith-considerable pressure through rubber rolls such, for instance, asthose of an ordinary clothes wringer. When this was impractical due tothe shape of the piece, the silver was patted down with-a stiff wirebrush, starting the patting operation at one end and continuing to theother end. The object of such operations was to remove any entrapped gassuch as air bubbles from the joint. Such bubbles, if not removed, causedblisters to appear in the union and a consequent weakening thereof.

I have now found that any or all, as may be desired or as conditions mayrequire, of aforedescribed steps 2, 3 and 5 cane be eliminated byelectroplating mercury upon either or both of the surfaces to be united,thereby simplifying the process and lessening the time requiredtherefor, decreasing labor costs and reducing health hazards.

The process thenis essentially as follows, and for illustrative purposesthe bonding of metallic lllver to metallic copper will be described.

The copper parts are first thoroughly cleaned in any suitable way, forexample by an acid dip in the usual and well known manner, followed byrinsing to remove excess acid and foreign impurities. Cold water, forexample, is a suitable Jinsing fluid.

The cleaned copper surfaces are then electroplated with mercury, usingany suitable mercury plating solution. For purpose of illustration Imention below a plating solution which I have used successfully,although it is to be distinctly understood that other mercury platingsolutions may be employed and that the specified percentages may bevaried as desired or as conditions A solution made in the proportionsstated is put into an electroplating tank of ferrousmetal such as steel,which'serves as the anode. The copper parts to be mercury plated areplaced in this solution, the copper articles being the cathode. Currentis applied in well known manner using, for example, a current density offrom about 10 to 15 amperes per square foot. A uniform coating ofmetallic mercury is plated on the copper in from about 3 to 5 seconds.It is simply necessary to secure complete coverage, as any excessmercury merely flows oif. If desired, surfaces not to be joined toanother metallic member, and which therefore it would not be necessaryto mercury plate in carrying the present invention into effect, may bemasked to prevent mercury plating thereof. It is more economical andless timeconsuming, however, to plate the entire work,

since the excess mercury may be volatilized during a subsequentoperation.

After the electroplating of the mercury on the copper the piece isrinsed, for example by dipping in cold water followed by hot water, and

then dried, for example by subjecting to a hot air blast for a fewseconds, usually not more than about 5 seconds.

A silver strip is now placed on the mercuryplated copper surface at thepoint of desired permanent union. If desired, this silver strip may beelectroplated with mercury or it may be amalgamated with mercury merelyby rubbing with mercury or immersion in a bath of :mercury. I have foundthat when the copper surface is electroplated with mercury it is usuallynot necessary to coat the silver strip with mercury. However, undercertain conditions, for example when the silver strip is not perfectlyfiat, it is sometimes advantageous to coat both the silver and coppersurfaces with mercury.

As a matter of practical convenience in holding the silver striptemporarily in place at the point where it is to be joined to thecopper, I apply a volatilizable adhesive in the general locality wherethe silver is to be applied. For this purpose various alcohols such aspolyhydric alcohols and other viscous high-boiling organic substancesmay be used. More specific examples of suitable adhesives are glycerine,ethylene glycol, diethylene glycol, alkyl ethers of diethylene glycol,and the like. I have found glycerine to be particularly adapted for thispurpose. Glycerine or like or equivalent substance functions mainly asan adhesive to hold the silver strip in place while the combined articleis being placed in a press for subsequent treatment. Materials such asglycerine are not required as an aid to bonding the metals. Glycerine orthe like, however, does have a beneficial effect as a protective coatingfor the mercury-plated surface in case there is unavoidable delay of,for example an hour or so, before the assembled articles are subjectedto the final pressing operation.

The assembled parts are now placed in a press having heated platens.Suflicient work is spread on the platen so that the pressure will beuniformly distributed and will not crush the copper parts. Steel spacersare used at the sides of the pieces to prevent crushing, that is,deforming, the cop per. Advantageously a cushion of compressiblematerial, for example, a sheet of aluminum, pressboard (about 1/64 inchthick), or heavy paper, is placed over the entire area of the assembledparts. If desired, small individual pieces of such compressible materialsufliciently large to cover the silver, at the point where it is to befirmly united to the copper, may be used. Such a cushion compensates forirregularities in the copper surface and insures intimate contact of thecopper and silver throughout their entire adjoining surface areas.Without a cushion of the kind described, the copper might have to becompressed to its flow point to get uniform contact with the silver. Formost purposes, such deformation of the copper is undesirable. Foreconomic and other reasons a pressure no higher than necessary to obtaina strong bond is preferred.

The temperature of the platens is maintained below about 290 C.,advantageously between about 175 and 250 C., by any suitable means, forinstance, by electric heating units. Temperatures above about 290 C.have a harmful effect upon the copper, for example, detrimentallyaffecting its hardness. The purpose of applying heat is to remove excessmercury and to assist in the formation of a strong union between thesilver and the copper. Although the use of simultaneous heat andpressure is preferred, recent observations have led me to believe that aunion may be obtained by the use of pressure alone in bonding themercury-coated surfaces, in which case the work advantageously may beaged at room temperature for at least a day prior to use. For mostpurposes, however, it is desirable that the work be heated'to atemperature between about C. and 290 C. while simultaneously applyingpressure thereto.

The particular pressure employed depends upon the surface irregularitiesof the copper and the sufllcient to weaken it materially. Dependinginmost cases are effective in producing the desion treatment is notnecessary when the mercury sired results and are generally satisfactoryin bonding silver to copper by the above-described. process. The periodof time the'united parts are subjected to pressure depends to a largeextent upon the size of the individual pieces. For

the same size and kind of work, the time the work is under pressure issubstantially less when the procedure described in the presentapplication is followed as compared with the process set forth in myco-pending application Serial No. 90,882.

Usually it is necessary to leave the work in the press merely longenough to bring the work toa point within the preferred temperaturerange. Of course if no heat is employed during the pressure treatment,it may be necessary to subject the work to pressure for a longer periodof time. To decrease the health hazard, the press should be providedwith suction orifices at both sides thereof and in front for withdrawingmercury as it is vaporized from between the joint.

After pressing, the bonded pieces are removed from the press. in myco-pending application Serial No. 90,882, it is usually desirable toplace the work, after the pressing operation, in an oven and thereinheat it at a temperature of at least about 190 C. for 20 minutes orlonger-in order to remove excess mercury. Such heat treatment after acompresis electroplated upon the surface of a metal to be united, asherein described, to another metal since, for one reason, the mercuryfilm which is electrolytically deposited on the metal surface is so thinthat the excess mercury is readily distilled off during the simultaneousheat and pressure treatment. The plating operation, which is anessential step in carrying the present invention into effect, provides amore uniform coverage of mercury on the work than otherwise may beobtained. In addition, the mercury probably penetrates further withinthe surface layers of the work. It has been demonstrated under practicalconditions of operation that an electrolytically applied coating or filmof mercury on a metal I surface better protects such surface from oxygen(air), dust, lint and other foreign impurities than a mercury coatingproduced on metal by rubbing with, or immersion in, for example metallicmercury. Such contaminants, when present between I facing surfaces ofthe metals being united, are a cause of blister formation in thefinished joint and of defective work.

From the foregoing it will be noted that an electrolytically depositedcoating of mercury has properties different from, or in addition to,mercury coatings, obtained by merely rubbing an amalgamatable metalwith, or immersion of such metal in, mercury. A mercury-plated surfacetherefore is neither the same as, nor the equivalent of, amalgamatedmetallic surfaces otherwise produced.

Another embodiment of my invention com- In a process such asdescribedtion to, or in lieu of, electroplating the intermediatemetallic layer with mercury, the metals to be joined may be soelectroplated. In all cases the metals to be united must be capable ofbeing electroplated by, or of amalgamating with, mercury. By means ofpressure, preferably pressure and heat, a bond is formed which varies instrength with the metals other than mercury which are employed.

Lead, tin, silver, gold, copper, cadmium, and alloys thereof, arementioned as examples of metals which may be electroplated with mercuryand the mercury-plated metal then used to bond the same or'othermercury-plated, or amalgamated (by rubbing with, or immersing in,mercury) or non-mercury-covered articles, with varying strength of theresultant bonds. For uniting copper articles, sheet silver .is apreferred bonding metal because of the high tensile strength of theresultant bond.

Unions or joints produced in accordance with the present invention arepractically as strong,

and .of the same general physical structure as joints made as describedin my aforesaid copending application. The process herein described,however, is simpler, requires less time and involves less labor cost. Italso comprises less health hazard to workmen since hand application ofmercury to the metal surfaces is eliminated. 7

The terms fmtal, metals, and metallic as such metals at the point whereit is subsequently to be united to the other, assembling the metals inthe position of ultimate rigid union to bring facing surfaces inintimate contact with each other, and subjecting the resultant assemblyto pressure sufiicient to form a firm and permanent union of saidmetals.

2. A method of uniting metals having surfaces that can be electroplatedwith mercury which comprises applying electrolytically a coating ofmercury upon a clean surface of at least one of Such metals at the pointwhere it is subsequently to be united to the other, assembling themetals in the position of ultimate rigid union to bring facing surfacesin intimate contact with each other, and subjecting the resultantassembly to a simultaneous pressure and heat treatment sulficient toremove excess mercury and to form a firm and permanent union of saidmetals.

3. A method of uniting metallic copper members which compriseselectrolytically depositing a film of mercury upon clean surfaces ofsaid' members where subsequently united, bringing a thin sheet ofmetallic silver into contact with the mercury-plated surfaces of thecopper members at the point of ultimate rigid union, and

film of mercury upon clean surfaces of said members where subsequentlyunited, placing a thin mercury-coated sheet of metallic silver betweenand in contact with the mercury-plated surfaces of the copper members atthe point of ultimate rigid union, and subjecting the resultant assemblyto a simultaneous pressure and heat treatment sufiicient to removeexcess mercury and to form a strong, permanent union between the coppermembers.

5. A method of uniting metallic silver and copper members whichcomprises applying an electrolytic coating of mercury upon a cleansurface of a copper member, bringing a. silver member into contact withthe mercury-plated surface of the copper member, and subjecting theresultant assembly to compression between about 1,000 and 40,000 poundsper square inch.

6. A method of uniting silver and copper arti cles which comprisesproviding a surface of a copper article, at the point of subsequentrigid union with a silver article, with an electrolytic coating ofmercury, bringing the silver article into contact with themercury-plated surface of the copper article, cushioning the assembledarticles with a compressible material to compensate for any surfaceirregularities therein, and subjecting the assembly to a simultaneousressure and heat treatment, said pressure being between about 1,000 and40,000 pounds per square inch and said heat being between about 100 and290 C.

WILLIAM J. WEDER.

